The invention relates to a superconductive element containing Nb3Sn, in particular a multifilament wire, comprising at least one superconductive filament which is obtained by a solid state diffusion reaction from a preliminary filament structure, said preliminary filament structure containing an elongated hollow pipe having an inner surface and an outer surface, wherein said hollow pipe consists of Nb or an Nb alloy, in particular NbTa, wherein the outer surface is in close contact with a surrounding bronze matrix containing Cu, Sn and possibly other elements, and wherein the inner surface is in close contact with an inner bronze matrix also containing Cu, Sn and possibly other elements.
A superconductor of this type is known from GB 1 543 149 A.
Superconductive Nb3Sn wires are typically produced by the powder in tube process (PIT-process), by the internal Sn diffusion method, or by the bronze route.
In the bronze route, a number of niobium (Nb) rods are inserted into a copper (Cu) and tin (Sn) containing bronze matrix. By repeated extruding, bundling and insertion into further bronze cans, a ductile wire with numerous Nb fibers embedded in a bronze matrix is obtained. Some pure copper is also introduced into the wire in order to improve its thermal conductivity. The wire is then brought into the desired shape, e.g. by winding the wire into a coil. Subsequently, the wire is annealed at a temperature of about 600-700° C. During this solid state diffusion reaction, Sn originating form the bronze diffuses into the Nb fibers and forms Nb3Sn, which has superconductive characteristics. The Nb3Sn phase is also called A15 phase. The Nb3Sn filamentary bundles show diameters of 4 to 5 μm.
The so produced superconductive Nb3Sn wires show high mechanical stability, small effective filament diameter, <20 μm and high homogeneity for very long lengths (well above 3 km).
Typically the Nb fibers do not completely react to Nb3Sn, but some Nb remains unreacted within the filaments. Within the Nb3Sn phase, a concentration gradient yields, varying from 25 atomic % Sn content near the boundary to the bronze matrix to 18% Sn content near the unreacted Nb core.
The Cu—Sn/Nb composite billet is fabricated by assembling a certain number of rods, consisting of a Nb core inside a Cu—Nb tube, surrounded by an external Cu—Sn can. The deformation occurs by extrusion and wire drawing. The obtained hexagonal rods are bundled and again inserted into a Cu—Sn can, followed by a second extrusion and hexagonal wire drawing. A third bundling and extrusion is followed by wire drawing to the final wire diameter, of the order of 1 mm. At this point, the Nb filament is around 5 μm, the total number of filaments reaching up to 10,000.
Nb3Sn with low Sn content exhibits inferior superconductive properties, in particular a low critical temperature Tc and low critical magnetic field strength Bc2. Therefore, high and homogeneous Sn contents in the Nb3Sn phase are desired. The Sn content in the Nb3Sn phase can be increased by increasing the annealing temperature (=reaction temperature) and/or the annealing time (=reaction time). However, this also induces accelerated grain growth, which deteriorates the superconductive properties of the filament again.
The described Bronze route process is well established and is at the present day the fabrication method covering the largest part of the market. However, recent important progress in the two other techniques, the “Internal Sn” process and the Powder-In-Tube (or PIT) process has created a new situation: a further improvement of the critical current densities of bronze route Nb3Sn wires is mandatory to remain competitive in the market. The necessity of such an improvement is also illustrated by the fact that the costs of bronze route wires may exceed those of Internal Sn wires by a factor two.
In GB 1 543 149 A, a method for producing a Nb3Sn based superconductive wire is described. It is proposed to use a Nb tube and arrange a bronze matrix within said tube and a bronze matrix outside said tube. This arrangement is subjected to an extension and wire drawing process. During a thermal treatment, Nb3Sn is formed both on the inner surface of the Nb tube and the outer surface of the Nb tube. By this means, Sn is introduced into the Nb material from two sides, improving the Sn supply.
However, it has been found out that wires produced by the method described in GB 1 543 149 A cannot be used in superconducting magnets due to a poor mechanical stability:
In their publication “Properties of Multifilamentary Nb3Sn Supercondutors Fabricated by the Internal Bronze Approach”, Advances in Cryogenic Engineering, Vol. 26, Plenum Press, pp. 451-456, New York (1980), R. M. Scanlan et al. state:    “The highest critical current densities in the Nb3Sn layer obtained by the internal bronze approach were about 600 A/mm2 (at 12 T), compared with about 2500 A/mm2 (at 12 T) for the external bronze approach. This lower value is believed to be due to the higher strains produced in the Nb3Sn layer in the internal bronze configuration.”
After annealing, the superconductive wire shrinks during cooling. Bronze has a relatively high thermal expansion coefficient αbronze of 17*10−6K−1, compared to Nb3Sn with αNb3Sn of 9*10−6K−1. Therefore, bronze shrinks more than the Nb3Sn phase (which originated from the Nb tube) during the final cooling. The outer bronze matrix exerts a radial compressive stress onto the Nb3Sn phase, whereas the inner bronze matrix within the Nb3Sn phase exerts a tensile stress onto the Nb3Sn phase. As a result, the Nb3Sn phase undergoes shear stresses which destabilize the superconductive filament. The Nb3Sn phase can peel off the bronze matrices and crack. For these reasons, the “double bronze technique” described in GB 1 543 149 A was rejected by the experts in the field and has never been used in commercial applications.
In their publication “Development of Bronze-Processed Nb3Sn Superconducting Wires for High Field Magnets”, IEEE Transaction on Applied Superconductivity, Vol. 12, No. 1, pp. 1045-1048 (2002), G. Iwaki et al. describe a preliminary filament structure with a Nb tube, an inner tantalum matrix and an outer bronze matrix is described. During annealing, at the outer surface of the Nb tube a Nb3Sn phase is formed.